Four hundred thousand new cases of congestive heart failure are diagnosed in the United States annually, a number which will only rise in the foreseeable future with the aging of the baby-boom generation. According to the Framingham Heart Study, the five-year mortality rate for patients with congestive heart failure was 75 percent in men and 62 percent in women. Standard medical and surgical therapies benefit only a small percentage of patients with ventricular dysfunction. Potential cardiac transplant recipients with hemodynamic instability may receive temporary mechanical circulatory support, such as an implantable blood pump, as a bridge to cardiac transplantation. Moreover, estimates in the field suggest that 17,000 to 66,000 patients each year in the United States may benefit from a permanent implantable blood pump.
The ventricular assist device (VAD) is a blood pump designed to assist or replace the function of either ventricle, or both ventricles, of the heart. A right ventricular assist device (RVAD) supports pulmonary circulation by receiving or withdrawing blood from the right ventricle and returning it to the pulmonary artery. A left ventricular assist device (LVAD) supports systemic perfusion by receiving or withdrawing blood from the left ventricle (or left atrium) and returning it to the aorta. A biventricular assist device (BVAD) supports both ventricles of the heart. Ventricular assist devices may be either implantable or extracorporeal, with implantable VADs positioned intracorporeally in the anterior abdominal wall or within a body cavity (other than the pericardium) and with extracorporeal VADs located para corporeally, along the patient's anterior abdominal wall, or externally at the patient's bedside.
The first ventricular assist devices attempted to mimic the pulsating flow of the natural left ventricle by utilizing flexible chambers with volumes approximately equal to the volume of the respective ventricle being assisted. The typical volume of blood expelled by the left ventricle of an adult is between 70–90 ml, but may range from 40–120 ml. The chambers are expanded and contracted, much like a natural ventricle, to alternately receive and expel blood. One way valves at the inlet and outlet ports of the chambers ensured one way flow therethrough.
So-called “pulsatile pumps” may include one or a pair of driven plates for alternately squeezing and expanding flexible chambers. The flexible chambers typically comprise biocompatible segmented polyurethane bags or sacs. The blood sac and drive mechanism are mounted inside a compact housing that is typically implanted in the patient's abdomen. A controller, backup battery, and main battery pack are electrically connected to the drive mechanism. Even the most basic drive mechanisms of the prior art are relatively complex and expensive, and typically incorporate some type of mechanical cam, linkage, or bearing arrangement subject to wear.
Because of the varying volume of the blood sac within the rigid encapsulation housing of pulsatile pumps, accommodation must be made for the air displaced thereby. Some devices utilize a percutaneous tube vented to the atmosphere, which is a simple approach but has the disadvantage of a skin penetration and associated infection risk. Another approach, proposed for fully-implantable VAD systems, is to use a volume compensator. This is a flexible chamber, implanted in the thoracic cavity adjacent to the lungs and communicating with the air space within the housing and outside the blood sac via an interconnecting tube. As the blood sac expands with incoming blood, air is displaced from the housing to the volume compensator. Conversely, expulsion of blood from the blood sac creates a negative pressure within the housing and pulls air from the volume compensator. While eliminating the infection risk of the percutaneous vent, the volume compensator poses certain challenges: increased system complexity, an additional implanted component and potential site of infection, maintaining long-term compliance of the implanted volume compensator sac, problems associated with gas diffusion in or out of the enclosed volume, and problems associated with changes in ambient pressure, such as experienced during a plane flight.
One example of an electric pulsatile blood pump is the Novacor N100 Left Ventricular Assist System (World Heart Inc., Oakland, Calif.). This system contains a single polyurethane blood sac with a nominal stroke volume of 70 ml that is compressed by dual symmetrically opposed pusher plates in synchronization with the natural left ventricle contraction. The pusher plates are actuated by a spring-decoupled solenoid energy converter. The blood pump and energy converter are contained within a housing that is implanted in the patient's abdomen. The N100 employs a percutaneous vent tube that also carries power and control wires.
An example of an electric pulsatile blood pump not requiring external venting is disclosed in U.S. Pat. No. 6,264,601 (“the '601 patent”), incorporated herein by reference. The system of the '601 patent has two pumping chambers formed from two flexible sacs separated by a pusher plate, with the sacs and pusher plate contained within one housing. An electromagnetic drive system acts on an iron armature surrounded by a cylindrically symmetric permanent magnet within the pusher plate to alternatively pump blood through the two sacs by compressing one sac and then the other against the housing. Since each sac contains only fluid that is alternately received and discharged as the pusher plate reciprocates, the total volume of the pump remains constant during pumping and no venting or volume compensator is required. The input and output of each sac includes a one-way valve, providing unidirectional flow that pumps the fluid in a preferred direction. The most efficient use of the electromagnetic drive system is achieved when the power and energy required in each pump stroke is approximately equal.
The '601 patent describes several alternative arrangements for using a blood pump, including a left or right VAD that couples the input and output flows from each chamber in either parallel or series, and a BVAD that separately uses two separate VADs to assist the left and right ventricle. One embodiment described in the '601 patent is a series-displacement pump, in which a first chamber receives a fluid for pumping, and provides that fluid to the input of a second chamber for further pumping (“the '601 series-displacement pump”). In operation, the '601 series displacement pump alternates between a pump stroke and a transfer stroke. When used as a VAD, the pump stroke pumps blood from the second chamber into the aorta while blood is drawn from the ventricle into the first chamber. In the transfer stroke, blood from first chamber is transferred to the second chamber. The fluid connection between the chambers is an external transfer conduit that connects the output of the first sac to the input of the second sac.
The '601 series-displacement pump has several advantages over other prior art pumps including, but not limited to, the ability to provide pulsatile flow, the use of fewer blood conduits and valves, and reduced size. However, the electromagnetic drive system of the '601 patent is optimized for bi-directional use, while the power and transfer strokes of the '601 series-displacement pump each have different power and energy characteristics. While the pump of the '601 patent is capable of operating as a series-displacement pump, there are energy losses that result from not having the drive and pump matched for series operation. Also, in general, the pump of the '601 patent includes a permanent magnet to drive the pusher plates that has a radially symmetric design that is expensive and difficult to manufacture.
Series-displacement pumps generally provide fluid communication between chambers through external conduits. Examples of series-displacement pumps using external conduits include the '601 series-displacement pump, and the pump-driven diaphragm pump and a pusher plate driven pump between two variable-volume chambers as described in U.S. Pat. Nos. 4,468,177 and 4,547,911 to Strimling. The Strimling devices are similar to the '601 pump in that each of the Strimling devices may function either as a BVAD heart pump with each chamber communicating separately with a respective ventricle of the heart, or as a single ventricle-assist pump wherein the two chambers are connected in series with a shunt therebetween.
In recent years there has been increased study into the potential of using rotary pumps (centrifugal or axial) for ventricular assist. These pumps employ fast-moving impellers to impart forward flow to the blood. The impellers are either supported by bearings or are magnetically levitated. A significant advantage of rotary pumps is their relatively compact size and low cost. In addition, the pressure difference maintained by the impeller eliminates the need for one-way valves as in pulsatile pumps. Finally, no venting or volume compensator is necessary.
The use of rotary pumps has generated a significant amount of interest in this field, but as yet many drawbacks prevent general acceptance. For instance, bearing-supported impellers usually require lubrication that must be absolutely kept out of contact with the blood, thus requiring seals that remain highly effective for extended periods. In some designs, the bearings are within the pump housing in contact with blood, which is then used as the lubricating fluid and may be subject to degradation. In addition, the heat generated by some bearing configurations may adversely affect the blood. Some designs eschew bearings altogether and instead utilize magnetically levitated impellers. However, these are relatively complex and sometimes unstable.
A safety issue with rotary pumps is their non-occlusive character, which provides a shunt path for blood regurgitation if the impeller is not rotating. That is, the one-way valves in pulsatile pumps ensure a uni-directional pathway through which blood is propelled, and prevent regurgitation from the arterial vessel if the device shuts off or fails. The natural ventricle can thus function as a back-up perfusion system, bypassing the pump circuit. If the impeller in a rotary pump stops, however, a flow path is created permitting blood from the arterial vessel to be shunted through the pump back into the ventricle, thus seriously impairing the back-up capability of the natural ventricle. To prevent this situation, a one-way valve or occluder of some sort must be provided at the rotary pump outflow. A still further issue with rotary pumps, as yet to be resolved, is the efficacy of the continuous flow of blood provided thereby. There is considerably less experience in the use of long-term circulatory support with continuous flow pumps as opposed to a vast body of experience with pulsatile flow pumps.
In view of the foregoing, there is an ongoing need in the art to improve upon conventional ventricular assist devices, and in particular upon series-displacement pumps. For example, reductions in size and the reduction of weight of the drive units would be advantageous to facilitate full implantation of a device. In addition, it would be advantageous to more closely match the power and operating speeds of pulsatile, series-displacement pumps to provide efficiently use of power over the cardiac cycle while providing pumping during systole. Further, a device that is low in cost but does not have the disadvantages of rotary pumps would be advantageous for long-term use. Accordingly, there remains a need in the art for a small, efficient, atraumatic, and fully implantable series-displacement ventricular assist device that overcomes the deficiencies of conventional devices.
Therefore, it is one aspect of the present invention to provide a ventricular assist device that is smaller, more robust and more efficient than prior art ventricular assist devices.
It is another aspect of the present invention to provide a ventricular assist device driven by an electromagnetic device at physiological speeds with high pump efficiency.
It is one aspect of the present invention to provide an electromagnetic drive useful for a ventricular assist device that has constant force characteristics, and that produces a pressure under rest conditions.
It is yet another aspect of the present invention to provide an electromagnetic drive useful for a ventricular assist device that produces a force that varies approximately linear with the coil current of the drive.
It is yet another aspect of the present invention to provide an electromagnetic drive useful for a ventricular assist device that can be easily controlled to produce desired output pressures.
It is another aspect of the present invention to provide a ventricular assist device that nominally beats once per heart beat using an electromagnetic drive that is optimized for efficiency and weight.